It is stumbling only under hard acceleration and while driving

[johnnycharger]

Hi Guys
Got my charger back on the road but it is having a stumbling issue. It started stumbling only if I mash the pedal and only while in gear driving. If I have it in park I can rev the snot out of it and it is fine. I noticed gas leaking from the front driver's side of the carb so I replaced the accelerator pump diaphragm which fixed the leak but not the stumbling. Any guesses?

[Kern Dog]

I have seen this happen when the ignition system is not working 100%. The increased demand under a load resulted in misfires.
My brother had a slant six '78 D 100 that idled great but popped and stumbled when in gear while accelerating. New spark plug wires changed everything.
This may not be your problem but it is worth a look.
Good luck...Report back with whatever happens.

[XH29N0G]

I had the same experience as KD with ignition related issues, but since you also said you messed with the accelerator pump, I would also check that out.  The key is that you know the car was dialed in before you put it up for a rest.

[Canadian1968]

First thing first. Make sure the accel. pump is adjusted correctly !

The car was running fine before the pump replacement?

[johnnycharger]

Hi guys
Thanks for the suggestions. Car was parked for a bit and ran great before. Just before finishing the front end rebuild the fuel pump died. I replaced that as well as my spark plugs, fuel filter and then the accelerator pump once I saw it leaking. I will double check the plug wires are on right and I will check the adjustment of the accelerator pump.

[XH29N0G]

A quick question.  After you mash the pedal and it stumbles, does it recover and accelerate, or does it just stumble.

[johnnycharger]

It does not recover. If I don't let off I am certain it will stall.

[XH29N0G]

IF it doesn't recover, I think that might be a clue and someone else here will be better at diagnosing than me.  Do you know that the right amount/pressure of fuel is making it to your carburetor?  

You mention replacing the fuel pump.  Was it a mechanical one? If so, did you check the pushrod when you replaced it?  

I had the pushrod wear down on my fuel pump and the car would just die when I applied gas.  I had an A/F gauge so I had a clue about why.  To confirm, I installed a fuel pressure gauge into the system, taped it to the windshield and drove around.  Wait to see if someone else has a recommendation.

[Kern Dog]

Over the summer I had the car down for a new heater and A/C system installation. I was doing different things as well so the whole deal took a few months. By October the car was real hard to start and keep running so I suspected the carburetor was to blame after switching distributors, coils, electronic ECUs did no good. After a carburetor rebuild, I drained the gas and added new 91 octane gas. The car ran fine afterwards so I suspect that all along, the gas had just gone bad. I had a full tank in November 2018 and hadn't driven the car much in 2019. The California gasoline does NOT stay potent for that long. My lower compression engines will run on old gas but my Charger seems to need a fresh mix to run right.

[johnnycharger]

IF it doesn't recover, I think that might be a clue and someone else here will be better at diagnosing than me.  Do you know that the right amount/pressure of fuel is making it to your carburetor?  

You mention replacing the fuel pump.  Was it a mechanical one? If so, did you check the pushrod when you replaced it?  

I had the pushrod wear down on my fuel pump and the car would just die when I applied gas.  I had an A/F gauge so I had a clue about why.  To confirm, I installed a fuel pressure gauge into the system, taped it to the windshield and drove around.  Wait to see if someone else has a recommendation.

Good point. I did measure the push rod while it was out because I recalled hearing about that.

[johnnycharger]

Over the summer I had the car down for a new heater and A/C system installation. I was doing different things as well so the whole deal took a few months. By October the car was real hard to start and keep running so I suspected the carburetor was to blame after switching distributors, coils, electronic ECUs did no good. After a carburetor rebuild, I drained the gas and added new 91 octane gas. The car ran fine afterwards so I suspect that all along, the gas had just gone bad. I had a full tank in November 2018 and hadn't driven the car much in 2019. The California gasoline does NOT stay potent for that long. My lower compression engines will run on old gas but my Charger seems to need a fresh mix to run right.

That is true our gas does suck. I don't think my problem at the moment is bad gas though because it runs fine with no load on it. I can rev it up like an 18 year old with a mullet and it doesn't skip a beat....

[metallicareload99]

Is the check valve for the accelerator pump installed, and did you use the bowel gaskets with the passage in it for the accelerator pump?

[johnnycharger]

I only swapped out the rubber gasket part with the metallic disk.

[johnnycharger]

The pump was set too tight. I set it correctly and hopefully that will solve it. I can test drive her because it is raining now.

[XH29N0G]

Sounds good. 

[johnnycharger]

Still is the same 🤔

[XH29N0G]

Some more questions. 

How long did the car sit since the last time it ran right?

What type of carburetor do you have?

Are there any other symptoms?

I think if you see a good healthy pulse of gas from the accelerator pump nozzle into the barrels that starts as soon as the throttle is pushed, then that should be OK. 

[johnnycharger]

Some more questions. 

How long did the car sit since the last time it ran right? Car was parked for over a year. I ran it every month and it always fired up instantly. Still does.

What type of carburetor do you have? Holley 750 double pumper

Are there any other symptoms? No. Car starts right away. Sounds great if l rev it in the driveway. Fails consistently under hard acceleration.

I think if you see a good healthy pulse of gas from the accelerator pump nozzle into the barrels that starts as soon as the throttle is pushed, then that should be OK.  Looks like it is indeed spraying a lot of gas when I push on the pump.

I really appreciate the help.

[XH29N0G]

I asked about how long it sat as a long shot that something else like varnish might be blocking the jets.  The things I can think of are:

1. The ignition or gas as KD suggests. 

2. Something with the carburetor.  For carburetor tests, I would check easy things first.
    2.a. You say the accelerator pump works, but a mechanical secondary, has two accelerator pumps and two squirters. Are both working? 
    2.b. After that, I might try disconnecting the linkage between secondaries and primaries so it runs only on the primaries and seeing if there is any change.
          If there is a change, then I would consider focusing on something with the carburetor and probably the secondaries. 
          If not, I would verify that enough fuel is making it to the carburetor, check float levels, and fuel pressure.  Then I would start looking again to ignition and check everything I   
          could think of that might be a culprit (gas, vacuum, timing - do you run a vacuum advance or mechanical advance?  And are you using electronic ignition or points?). 

It is not difficult to pull the bowl and metering plate off to see if there is anything obvious that was blocked with the passages, but you will need gaskets, and I would hold off. I'm hoping it will be something simple because I find the more things I try, the more likely I am to create another problem. 

[johnnycharger]

Thank you so much for the suggestions. I will try some today.
I also re read the instructions on adjustment of the accelerator pump and think that I may have got it wrong. I missed the step of disengaging the fast idle cam lever. So I am going to start by trying to figure out how to do that.
I really appreciate the help and support while I try to sort this out.

[lukedukem]

Hi Guys
Got my charger back on the road but it is having a stumbling issue. It started stumbling only if I mash the pedal and only while in gear driving. If I have it in park I can rev the snot out of it and it is fine. I noticed gas leaking from the front driver's side of the carb so I replaced the accelerator pump diaphragm which fixed the leak but not the stumbling. Any guesses?

what gear are you in. this might sound dumb but if your transmission kickdown isn't working properly and you mash it in 3rd gear you flood the engine with gas and if your not up to higher rpm then it boggs it down. hope that sounds right. if you try to drive it manually does it still do it?
I didn't ask if its and auto or manual. if its manual then ignore this. wild hunch, might sounds stupid but i'd rather sound dumb then not say anything to help

Luke

[johnnycharger]

Hi Luke
It is an automatic. Good points. I will try manual shifting today and I will pay attention to if it is gear specific.

[johnnycharger]

Ok, so it is definitely more prevalent in 2nd and 3rd. I adjusted the kick down further and that didn't fix it. Did some YouTube searching and found out about the paperclip trick to check the vacuum secondary. I checked it and the paperclip did not move. I tried it by hand and it sounds really weird to me but I have not ever checked one before. Here is a video of the noise.
https://youtu.be/n1stpw523FE
I am thinking the diaphragm is bad.

[XH29N0G]

I am not sure if that sounds strange or not.  There is a diaphragm that will compress, and I believe it will do so when you move it like you did.  That could be the noise. 

I also do not think that the vacuum secondary not opening would cause it to fall flat.  My reasoning is twofold. First,  I can drive in a quite spirited ways after diabling the secondaries on my car. Second, I had an old two barrel car and it did not fall flat when I floored it. 

I wonder if the secondaries are not opening is a consequence of the issue you are facing rather than the cause of it.  I thought that secondaries open when the flow through the primaries is high enough to draw a vacuum on some orifice connected to that vacuum pot. So I would think with the engine stumbling that it is not pulling a lot of air and therefore not likely to open the secondaries fully if at all.

[Kern Dog]

I can rev it up like an 18 year old with a mullet and it doesn't skip a beat....

Nobody called you on this but I noticed it. Funny stuff!
Carburetors ......If you are willing to drive,  I'd let you try one of mine that was recently rebuilt. I also have a  ThermoQuad that was rebuilt a year ago but has been in a box since then. You'd have to drive past Sacramento to get it . Whether that is worth your time or not, that is up to you.